1. Field of the Invention
The invention relates to a CMOS thin film transistor and a display device using the same, and more particularly, to a CMOS thin film transistor in which there is substantially no difference between the absolute value for current mobility and the absolute value for threshold voltage in a P-type thin film transistor and a N-type thin film transistor, and a display device using the same.
2. Description of Related Art
Generally, circuits using complementary metal oxide semiconductor thin film transistors (CMOS TFT) are used to drive active matrix liquid crystal display (LCD) devices, organic electroluminescent (EL) devices and image sensors. However, the absolute value of the threshold voltage of a thin film transistor (TFT) is generally larger than the absolute value of a threshold voltage of a metal oxide semiconductor (MOS) transistor using a single crystalline semiconductor. Furthermore, an absolute value of the threshold voltage of a N-type thin film transistor is quite different from an absolute value of the threshold voltage of a P-type thin film transistor. For example, the threshold voltage of the P-type thin film transistor is −4 V when the threshold voltage of the N-type thin film transistor is 2 V.
Therefore, large differences in absolute values between the threshold voltage of the P-type thin film transistor and the threshold voltage of the N-type thin film transistor is not desirable for circuit operation. In particular, large differences in absolute values of the threshold voltages of P-type and N-type functions as a big barrier in reducing driving voltage.
For example, a P-type thin film transistor having a large absolute value of threshold voltage does not operate properly at a low driving voltage. That is, the P-type thin film transistor functions only as a manual device such as a resistor and does not operate quickly enough. The driving voltage needs to be high enough to operate the P-type thin film transistor as a manual device.
Particularly, a work function difference between the gate electrode and the intrinsic silicon semiconductor is decreased as much as −0.6 eV in cases where the gate electrode is formed of a material having a work function of 5 eV or less, such as, aluminum. Consequently, the threshold voltage of P-channel TFT is shifted to a negative value, and the threshold voltage of N-channel TFT approaches 0 V. Therefore, the N-type thin film transistor is generally in the on-state.
In the above state, it is desirable that the absolute value of the threshold voltage of the N-type thin film transistor is almost equal to that of the P-type thin film transistor. In case of conventional single crystalline semiconductor integrated circuit technology, the threshold voltage has been controlled using N or P-type impurity doping at a very low concentration of 1018 atoms/cm3 or less. That is, the threshold voltage has been controlled to an accuracy degree of 0.1 V or less by impurity doping having a concentration of 1015 to 1018 atoms/cm3.
However, shift in threshold voltage is not observed even if impurities are added to a concentration of 1018 atoms/cm3 or less in when a semiconductor which is not a single crystalline semiconductor is used. Furthermore, if the concentration of impurities is 1018 atoms/cm3 or more, the threshold voltage is rapidly changed, and conductivity becomes p-type or n-type since polycrystalline silicon has many defects. Added impurities are trapped by the defects and not activated since concentration of the defects is 1018 atoms/cm3. Furthermore, concentration of impurities is larger than concentration of defects, excessive impurities are activated, and conductivity type is changed to n or p-type.
In order to solve these problems, a P-type thin film transistor and a N-type thin film transistor are fabricated in such a way that the length of the channel of the P-type thin film transistor is shorter than the length of the channel of the N-type thin film transistor. U.S. Pat. Nos. 6,492,268, 6,124,603 and 5,615,935 disclose varying the length of N-type and P-type channels. However, there are problems even in the disclosed fabrication process because the process of forming thin film transistors with channels of different length is complicated.